1. Field of the Invention
The present invention relates generally to stringed musical instruments, and more particularly to stringed instruments that maintain relative tune during string tension adjustments.
2. Description of the Related Art
Stringed musical instruments create music when strings of the instrument vibrate at wave frequencies corresponding to desired musical notes. Such strings typically are held at a relatively high tension, and the musical note emitted by the string is a function of the vibration frequency, length, tension, material and density of the string. The natural frequency of the vibrating string is described by the following wave equation:f=(½L)(T/d)1/2 
In this equation, f is the natural frequency of vibration, T is the tension on the string, d is the density of the string (in mass per unit length), and L is the length of the relevant portion of the string. Stringed musical instruments typically include a plurality of musical strings arranged generally parallel to one another. Preferably, the strings are configured to emit different notes when caused to vibrate. During use, the musician may vary the frequency of the string by pressing down on the string at a certain point in order to vary the effective length of the string, thus correspondingly changing the natural vibration frequency. The emitted musical note changes with the change in vibration frequency. As indicated by the equation, the vibration frequency is inversely proportional to the length of the vibrating portion of the string; thus, as the musician effectively shortens the string, the frequency of vibration increases, and thus the pitch of the emitted musical note correspondingly increases.
Each string of a stringed musical instrument typically is tensioned in relative tune to the other strings in order to facilitate predictable playing of chords and scales. This state, commonly referred to as being “in tune,” means that the natural frequency of the strings vary from one another by a predetermined interval. For example, conventional tuning of a guitar is such that the string at the lowest frequency is tuned to E, and subsequent strings are tuned to A, D, G, B and E. As such, each string is five half steps (the smallest frequency individually used in the standard 12-tone scale) higher than the previous string, except the G to B interval which is 4 steps. Adding all of the intervals, there are 24 half steps, which is two octaves (12 half steps being one octave).
An octave is the musical interval at which the frequency of the upper note is exactly twice that of the lower note. The frequency of vibration of the low E string and the high E string of a guitar are such that the emitted musical notes are two octaves away from each other. As indicated by the equation, a frequency may be doubled by halving the length of a musical string when the tension and density of the string are held constant. Different approaches are used, depending on which factors are desired and kept constant. For example, in order for the low E string and the high E string to be two octaves apart in a guitar in which the string lengths are equal, the tension on the high E string must be 16 times that of the low E string, or the density of the high E string must be 1/16 that of the low E string, or a combination of tension and density differences must create a factor of 16 so that when the square root of the term T/d is taken the result is 4, which indicates quadrupling of frequency in accordance with a two octave interval.
In conventional musical instruments, such as guitars, the tension of the strings relative to one another does not vary dramatically, mostly because of practical concerns. For example, too much tension may cause a string to be especially subject to breakage; too little tension may result in a string being so slack that it may contact the instrument body or interfere with other strings when vibrating during play. Accordingly, typically the density (mass per unit length) of the strings varies widely between strings in order to obtain a set of strings having the desired natural frequencies. For guitars, strings are sold in sets of six, with each string being weighted to produce its particular desired frequency within desired tension ranges.
Typically, guitar strings are fixed to the guitar at one end and attached to rotatable tuning knobs at the other end so that each string may be tightened with a suitable tension. Each string typically has its own knob (also called a tuning key). Stringing a guitar involves affixing one end of each guitar string to a mount on the body of the guitar, aligning the string in its place across the neck, and tightening and tuning the string by connecting it to its corresponding tuning key. Such stringing can be a time-consuming process.
Tuning a guitar is performed by turning each knob so as to tighten or slacken the string until the desired frequency is obtained. Tuning stringed instruments such as guitars can be time-consuming and difficult. Typically, a guitarist first correctly tunes the lower E string, and then progressively tunes the adjacent strings. For example, the E string is shortened (by pushing it against the guitar neck) to a position that produces an A note, and the adjacent A string is tuned by ear to match the A note as played on the E string. The D string adjacent to the A string is similarly tuned relative to the A string, as are the rest of the G, B and E strings progressively tuned relative to the adjacent strings. Such tuning by ear is typically very difficult for beginners and for those without a good sense of musical tones. Also, such tuning requires a reference note to start, and such reference note is usually provided by a different instrument, and it has a different timbre than does a guitar, thus further complicating tuning.
A piano typically contains about 220 strings. Typically, piano tuning is accomplished in much the same manner as a guitar tuning, and all 220 strings are adjusted relative to one another.
On occasion, a guitarist may desire to change the pitch of his instrument in order to play a particular song. This can be accomplished by using a device known as a capo, which wraps around the neck of the guitar and can effectively shorten the length of all of the guitar strings, thus increasing the frequency and correspondingly increasing the emitted pitch of all of the strings, while maintaining the strings in relative tune. However, this operation relatively shortens the neck of the guitar, which may be undesired. Also, the guitarist must change the position of his fingers along the neck to play chords and such. Thus, it can be desired to completely retune the guitar to a higher pitch. This typically necessitates retuning the low E string, then the A, D and so on, which is difficult and time consuming. It is thus impractical to retune a typical guitar during a playing session.